1. Field of the Invention
The present invention relates to a photographic printer and an exposure control method, or more in particular to a photographic printer for displaying an image on a liquid crystal panel, irradiating light from a light source (such as an LED) onto the liquid crystal panel and exposing the image on a photosensitive material by the light transmitted through the liquid crystal panel, and an exposure control method for such a photographic printer.
2. Description of the Related Art
An index print is well known in which in order to facilitate search for a photo taken in a developed negative film, a plurality of frame images are arranged in compressed form and printed in matrix.
As a technique for producing this index print by a photographic printer, what is called a liquid crystal photographic printer has been proposed, in which each frame image of a negative film is read by a scanner or the like, processed as an image in a predetermined manner and stored in a memory. Image data including several frames are each read and displayed on a liquid crystal panel at a predetermined timing, and the image data thus displayed are irradiated by light. The light transmitted through the displayed image are exposed on a printing paper and appropriately processed to produce an index print.
The characteristic of drive voltage versus light transmittance of the liquid crystal constituting a liquid crystal panel used for the above-mentioned liquid crystal photographic printer, however, changes with temperature, and therefore stable operation thereof is difficult to attain.
In view of this, a technique (JP-A-4-147247) using a liquid crystal panel having an open liquid crystal and a closed liquid crystal for detection has been proposed, in which the exposure light amount is controlled by feedback on the basis of the difference in the transmitted light amount between the open liquid crystal and the closed liquid crystal and the light amount transmitted through the open liquid crystal (i.e. the light amount from a light source).
In above-mentioned liquid crystal photographic printer, a method for displaying a halftone image on a liquid crystal and exposing the halftone image has been proposed.
However, the exposure light amount of a halftone image is difficult to control only on the basis of the light amount transmitted through full open and full closed liquid crystals as in the conventional method disclosed by the above-mentioned patent publication. Further, the relation between the drive voltage and the transmitted light amount (i.e., the characteristic of drive voltage versus transmitted light amount as shown in FIG. 8) of the liquid crystal panel changes with temperature, and this fact is not taken into consideration in the prior art.
In recent years, a light emitting diode (hereinafter referred to as the LED) has come to be used widely in place of the halogen lamp. The LED light source, as compared with the lamp light source, has the advantages that a very small amount of heat is generated when it is turned on, the lighting operation is easily controlled, and further it can be reduced in size. For these reasons, the LED has been used also for the liquid crystal photographic printer.
Despite these advantages, the disadvantage of the LED light source is that the light amount is liable to change with ambient temperature. In other words, the light amount change of the LED light source and the resulting light amount irregularities may deteriorate the quality of the photographic print.
Further, the open/close speed of each pixel of the liquid crystal panel is liable to change with ambient temperature, and the error of the open/close speed often causes an error of the time (exposure time) during which the printing paper is actually irradiated by light.
The present invention has been developed to obviate the above-mentioned problems and the object of the invention is to provide a photographic printer and an exposure control method capable of producing a photographic prints of stable quality.